Expose the ability to retrieve the result of a type validator via the SB API. To keep it simple, do not expose the pair, but just return a NULL string for success, and a non-NULL string for error; If we were to decide to expose the pair, we would need an SBTypeValidatorResult, which is fine, but it should come as part of exposing type validators through the SB API rather than as a one-off thing. So, KISS for now

git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@217299 91177308-0d34-0410-b5e6-96231b3b80d8